Anxiety disorders are one of the most prevalent diagnoses in youth, often resulting in impaired social and school functioning. Research on treatments for youth anxiety is primarily based in traditional clinical settings. However, integrating youth psychotherapies into the school environment improves access to evidence-based care. The present study is a pilot, randomized waitlist-controlled trial of a school-based, group acceptance and commitment therapy-based (ACT) intervention for adolescents with anxiety. Students at two separate schools (N = 26) with elevated anxiety were randomized to a 12-week waitlist or to immediate treatment. Participants in the immediate treatment condition reported statistically significant decreases in anxiety and class absences at post treatment and follow-up compared to the waitlist group. No statistically significant differences were found between groups for depression, psychological flexibility, positive mental health, and student well-being. However, medium within-condition effect sizes were seen in the treatment group for all outcomes. Participants reported the treatment as favorable with good acceptance ratings. Overall, this study supports ACT as a viable intervention for schools and other clinical settings providing services to adolescents with anxiety.